In the arterial wall, ATP:creatine N-phosphotransferase (E.C.2.7.3.2) is present in the form of its three isoenzymes, MM, MB, and BB, the latter being predominant. Six iodinated contrast agents in routine angiographic use were examined from the point of view of their effects in vitro on the isoenzymes of ATP:creatine N-phosphotransferase from the arterial wall, compared with their effects on the same isoenzymes in other organs. There is a clear inhibitory effect from the contrast agents on all organs, but it is least marked for the arterial wall. Furthermore, nonionic compounds have far less enzyme-inhibitory effect than ionic compounds.